The semiconductor industry has a market driven need to reduce the size of devices, such as transistors, and increase the device density on a substrate. Some product goals include lower power consumption, higher performance, and smaller sizes. FIG. 1 illustrates general trends and relationships for a variety of device parameters with scaling by a factor k. The continuous scaling of MOSFET technology to the deep sub-micron region where channel lengths are less than 0.1 micron (100 nm or 1000 Å) causes significant problems in the conventional transistor structures. For example, junction depths should be much less than the channel length. Thus, with reference to the transistor 100 illustrated in FIG. 1, the junctions depths 101 should be on the order of a few hundred Angstroms for channels lengths 102 that are approximately 1000 Å long. Such shallow junctions are difficult to form by conventional implantation and diffusion techniques. Extremely high levels of channel doping are required to suppress short-channel effects such as drain induced barrier lowering, threshold voltage roll off, and sub-threshold conduction. Sub-threshold conduction is particularly problematic in DRAM technology as it reduces the charge storage retention time on the capacitor cells. These extremely high doping levels result in increased leakage and reduced carrier mobility. Thus, the expected improved performance attributed to a shorter channel is negated by the lower carrier mobility and higher leakage attributed to the higher doping.
Leakage current is a significant issue in low voltage and lower power battery-operated CMOS circuits and systems, and particularly in DRAM circuits. The threshold voltage magnitudes are small to achieve significant overdrive and reasonable switching speeds. However, as illustrated in FIG. 2, the small threshold results in a relatively large sub-threshold leakage current.
Some proposed designs to address this problem use transistors with ultra-thin bodies, or transistors where the surface space charge region scales as other transistor dimensions scale down. Dual-gated or double-gated transistor structures also have been proposed to scale down transistors. As commonly used in the industry, “dual-gate” refers to a transistor with a front gate and a back gate which can be driven with separate and independent voltages, and “double-gated” refers to structures where both gates are driven when the same potential. An example of a double-gated device structure is the FinFET. “TriGate” structures and surrounding gate structures have also been proposed. In the “TriGate” structure, the gate is on three sides of the channel. In the surrounding gate structure, the gate surrounds or encircles the transistor channel. The surrounding gate structure provides desirable control over the transistor channel, but the structure has been difficult to realize in practice.
FIG. 3 illustrates a dual-gated MOSFET with a drain, a source, and front and back gates separated from a semiconductor body by gate insulators, and also illustrates an electric field generated by the drain. Some characteristics of the dual-gated and/or double-gated MOSFET are better than the conventional bulk silicon MOSFETs, because compared to a single gate, the two gates better screen the electric field generated by the drain electrode from the source-end of the channel. The surrounding gate further screens the electric field generated by the drain electrode from the source. Thus, sub-threshold leakage current characteristics are improved, because the sub-threshold current is reduced more quickly as the gate voltage is reduced when the dual-gate and/or double gate MOSFET turns off. FIG. 4 generally illustrates the improved sub-threshold characteristics of dual gate, double-gate, or surrounding gate MOSFETs in comparison to the sub-threshold characteristics of conventional bulk silicon MOSFETs.
FIGS. 5A-C illustrate a conventional FinFET. FIG. 5A illustrates a top view of the FinFET and FIG. 5B illustrates an end view of the FinFET along line 5B-5B. The illustrated FinFET 503 includes a first source/drain region 504, a second source drain region 505, a silicon fin 506 extending between the first and second source/drain regions. The silicon fin functions as a transistor body, where the channel between the first and second source/drain regions is horizontal. A gate insulator 507, such as silicon oxide, is formed over the fin, and a gate 508 is formed over the fin after the oxide is formed thereon. The fin of the illustrated conventional FinFET is formed over buried oxide 509. FIG. 5C illustrates a conventional etch technique for fabricating the fin for the FINFET. As illustrated in FIG. 5C, the fin width is defined by photolithography or e-beam lithography and etch. Thus, the fin width is initially a minimum feature size (1 F). The width of the fin is subsequently reduced by oxidation or etch, as illustrated by arrows 510.